1. Field of the Invention
Aspects of the present invention relate generally to an differential pair of transistors, and more particularly to compensation of temperature differences in the differential pair.
2. Description of Related Art
A differential amplifier is a commonly used building block of analog integrated circuits. The differential amplifier can be implemented using bipolar junction transistors (“BJT”) or field effect transistors, such as MOSFETs. A commonly used BJT differential amplifier includes a pair of transistors whose emitters are connected. The emitters of the BJTs may be biased by a common current source I, such that the collector currents for the two transistors sum to equal I. Assuming the transistors are matched and no differential signal is applied to the bases of the differential pair, an equal amount of current will flow through the two transistors. However, if a differential signal is applied to the bases of the two transistors, the bias current flowing through the collectors of the two transistors will be unequal. This collector current can be calculated as an exponential function of the differential signal and the thermal voltage, VT, where VT is equal to Boltzmann's constant, k, multiplied by temperature (in degrees Kelvin) divided by the charge of an electron, q. The equation can be expressed as I2/I1=e(Vdiff/VT), where I1+I2=I and Vdiff is the differential signal applied to the bases of the transistors.
Because the thermal voltage, VT, is proportional to absolute temperature, the bias currents flowing through the transistors also vary with temperature. However, for certain applications, it is undesirable for the bias currents to drift with temperature. The above equation suggests that if the first order derivative dVdiff/dT is equal to the derivative dVT/dT, the quotient Vdiff/VT would remain constant over temperature, and similarly, the quotient I2/I1 would remain constant over temperature. It follows that if the current source I is also held constant, then the absolute values of both I, and I2 would remain constant.
Therefore, it may be desirable to provide a circuit arrangement that maintains a constant collector current relative to variations in temperature.